For techniques for making your own filament out of nurdles or recycled plastic, see -->filament production.

The extruded filament refers to the fine diameter plastic that exits an extruder and is fundamental to the technique RepRap machines use to build plastic models: fused filament fabrication. Typically, the diameter of the filament varies between 1mm and 0.3mm, with 0.5mm typical for many users. The standard extruder produces filament using high pressure and heat to force molten plastic thru a very tiny hole.

Occasionally, some may refer to the plastic 3mm feed stock as filament as well.

Die swell and Stretching

As the plastic deforms and exits the hole, it may 'rebound' somewhat and produce, when not constrained by any other factors, a final diameter slightly larger than the hole. The resulting Die Swell varies by material, temperature, and the hole diameter. PLA tends to have very little die swell, while HDPE has greater - 0.75mm, or 0.8mm filament can exit a 0.5mm hole.

As a machine will rarely extrude filament without additional constraint, this die swell diameter does not entirely determine the final metrics that determine your printer quality. For example, if the head motion moves fast enough, the extruded filament will stretch, and result in a thinner than normal result.

One can use these principles to adjust within some margin print quality against print speed. A model will print about twice as fast with 0.7mm filament, as compared to 0.5mm filament. However, the finish and appearance will have greater detail with 0.3mm filament. There are limits though - if you extrude slower, and move faster, the filament will stretch and break beyond some threshold -- an undesirable result. At the other end, if extrude faster, and move slower, the filament will bulge, produce nodules and blobs -- another undesirable effect.

To tune for either high quality prints, or faster printing time, a minimum and maximum range of final thread rates can prove useful. The following table list some values that Nophead posted in the forums for possible viable ranges of extruder he had used, based on the properties of the material and the extrusion hole diameter:

(*) NOTE - Nophead indicated that trying to extrude 0.5mm PLA thru 0.4mm nozzle resulted in poor quality because PLA does not have much die swell.

TODO - adding to this table for actual measured rates might be good, if people are willing to volunteer their data.

G Code control of filament

The RepRap Darwin 3D G-Code uses M codes to define a power rate to the extruder motor. This technique does not always produce exact extrusion rates, and the 5D gcode, using a direct stepper or position sensitive extruder will produce higher quality printing.

E as length of extrusion

The Mendel 5D G-Code (Mendel_User_Manual:_RepRapGCodes) uses the 'E' axis as a fully independent axis for G1 motion. The number, in imperial or in metric, represents the length of extruded filament. Different machines can and usually do have different filament diameters, and thus, each machine should be tuned to the desired target final filament diameter desired.

Example: with a 0.5mm extruder, using metric units, the gcode "G1 E50.93" should extrude about 10mm^3 of material, depending on how much die swell there is, and how the machine has been calibrated. The extruder is calibrated during commissioning.